1. Field of the Invention
The present invention relates to a photoacoustic image generation apparatus and an acoustic wave unit, and more particularly, to a photoacoustic image generation apparatus, which when a test object is irradiated with a laser beam having a plurality of wavelengths, generates a photoacoustic image on the basis of photoacoustic signals detected with respect to the respective wavelengths, and an acoustic wave unit.
2. Description of the Related Art
Hitherto, for example, as disclosed in JP2005-21380A and A High-Speed Photoacoustic Tomography System based on a Commercial Ultrasound and a Custom Transducer Array, Xueding Wang, Jonathan Cannata, Derek DeBusschere, Changhong Hu, J. Brian Fowlkes, and Paul Carson, Proc. SPIE Vol. 7564, 756424 (Feb. 23, 2010), a photoacoustic image forming apparatus that forms an image of the inside of a living body using a photoacoustic effect has been known. In the photoacoustic image forming apparatus, a living body is irradiated with pulsed light such as a pulse laser beam. Body tissues absorbing energy of the pulsed light expand in volume inside the living body irradiated with the pulsed light, and thus acoustic waves are generated. It is possible to detect the acoustic waves using an ultrasonic probe or the like, and to form a visible image of the inside of the living body on the basis of the detected signal (photoacoustic signal). In a photoacoustic image forming method, acoustic waves are generated in a specific light absorber, and thus it is possible to form an image of specific tissues in the living body, for example, blood vessels.
Incidentally, many of body tissues have an optical absorption property varying depending on a wavelength of light, and generally, the optical absorption property is unique for each tissue. For example, FIG. 12 illustrates molecular absorption coefficients of oxygenated hemoglobin (hemoglobin combined with oxygen: oxy-Hb) which is contained in a large amount in an artery of a human and deoxygenated hemoglobin (hemoglobin not combined with oxygen: deoxy-Hb) which is contained in a large amount in a vein, depending on light wavelengths. An optical absorption property of an artery corresponds to that of oxygenated hemoglobin, and an optical absorption property of a vein corresponds to that of deoxygenated hemoglobin. There is known a photoacoustic image forming method of irradiating blood vessel parts with a light beam having two different types of wavelengths and of distinctively forming images of an artery and a vein (for example, see JP2010-046215A), using a difference in light absorptivity according to the wavelengths.
Herein, with regard to a variable wavelength laser, JP2009-231483A discloses that a laser beam having a desired wavelength is obtained by disposing an etalon or a birefringent filter as a wavelength selection element within an optical resonator and adjusting the rotation angle thereof. In addition, JP2000-105464A discloses that an etalon as wavelength selection means is disposed within an optical resonator and that the etalon is scanned at a constant speed. JP2000-105464A discloses that laser oscillation is performed only when a transmission wavelength of the etalon is consistent with longitudinal mode oscillation of a laser beam and that the oscillation of the laser beam is performed in a pulsed manner when a scanning speed of the etalon is increased.